The invention relates to a dental device for generating preferably high-frequency mechanical vibrations for a dental handpiece according to the preamble of claim 1.
The term high-frequency within the context of the present description and the claims is understood to be a frequency lying in the ultrasonic range. However, the invention can also be used in the audible frequency range and in the infrasonic range.
When using dental handpieces, which comprise a tool driven at high mechanical frequency, the loading of the device generating the high-frequency mechanical vibrations is closely dependent upon the damping of the tool by the respective tissue which is treated. This loading is dependent upon the complex contact geometry between tool and tissue surface as well as the pressure with which the dentist presses the tool against the tissue which is to be treated. If the handpiece operates with a coupling fluid acting between the tool and the tissue surface, then the loading of the device for generating vibrations is also dependent upon whether, and how much coupling fluid is supplied.
In order to guarantee a level of efficiency of the ultrasonic handpiece which can be used in practice, it is necessary for the dentist to vary the contact pressure of the tool (and therefore of the handpiece). Finally, only very imprecise tactile feedback is available to the dentist during this process, since the high frequencies, at which the tool is mechanically vibrated, cannot be perceived by the human ear or cannot be perceived with the desired precision.
Furthermore, the dentist cannot respond so quickly to rapid load changes in order to reliably rule out the occurrence of excessively high vibrations resulting in damage to the handpiece or tool, or there is danger of a decrease or cessation in the vibration going unnoticed by the dentist, so that the treatment effect is reduced or ceases altogether.
It is therefore the object of the present invention to develop a device for generating high-frequency mechanical vibrations for a dental handpiece in such a manner that a reliable automatic limiting of the vibration amplitude is ensured.
This object is attained according to the invention by a device having the features disclosed in claim 1.
Advantageous developments of the invention are disclosed in the subclaims.
In the vibration generating devices used in the field of dentistry, which usually operate in the ultrasonic range, it has been found that the supply current supplied to the actual vibration generator (usually a stack of piezoelectric disks) is a good measure of the instantaneous amplitude of the vibration generated. In the device according to the invention as disclosed in claim 2, this supply current is used as an actual value for an amplitude control, which not only prevents unduly high amplitudes, but also ensures a constant amplitude of the vibration generator and therefore also of the tool during operation under varying loading.
According to claim 3, a precise control of the amplitude is obtained by slight variations in the frequency of the oscillator. A steep control characteristic curve is therefore obtained. The development of the invention according to claim 4 also allows for the amplitude control in a particularly simple manner via a control signal, by means of which the control terminal of a controllable oscillator can be actuated. In this manner of proceeding, use is made of the fact that each frequency variation results in an amplitude variation on a flank of the resonance curve of the overall system, which is formed by the operating circuit, the vibration generator, the tool and the load, or more precisely, a frequency change in the direction of the maximum of the resonance curve results in an increase in amplitude, whilst a frequency change in the other direction results in an amplitude reduction. Controllable oscillators can also be obtained as economic and reliable components.
The development of the invention according to claim 4 is advantageous with a view towards a rapid adjustment of the actual amplitude to the nominal amplitude. In this manner, excessive mechanical loading of the parts of the device lying between the vibration generator and the tool, which could result in material failure, can be avoided.
The rapid limiting of the supply current supplied to the vibration generator can be realised in a particularly simple manner according to claim 5 in that the current limitation or a current interruption is effected directly upstream of the vibration generator, so that inertia of electronic logic circuits and power stages does not play a role.
The manner of realising the current limitation disclosed in claim 6 allows for a low level of loss during correct operation of the device.
The development of the invention according to claim 7 also promotes the immediate reduction of excessive supply current for the vibration generator. The variant according to claim 7 offers the advantage that the current limitation is effected via the control signal for the oscillator in a manner requiring a low level of power, so that it is not necessary to provide a current limiting circuit designed for higher currents.
Since, according to the invention, it is possible to avoid mechanical overloading of the vibration generator and the parts driven thereby even during varying load conditions, it is also possible to select a higher mean amplitude used for operating (not requiring any safety margin accounting for varying load conditions). However, this then results in higher mean load values. In the case of a vibration generator which comprises a plurality of piezoelectric disks resting against one another in a prestressed manner, it is therefore also of particular interest to also ensure reliable and safe contacting of the individual piezoelectric disks over long-term operation and also to prevent fatigue failure in the power connections to the piezoelectric disks. To this end, it is proposed in claim 8 to apply the electrode layers of the various disks directly onto the disks (by electroplating, evaporation, sputtering, etc.) instead of placing them as separate elements between the disks, as in conventional fashion.
In this respect, it is then particularly advantageous according to claim 8 if the various electrode layers each comprise a connecting section constructed on the cylindrical surface of the disks. In this manner, the connecting sections are particularly reliably protected against deformation and fatigue failure.
With the development of the invention according to claim 9, a symmetrical application of potential to the electrode layers is ensured.
In a device according to claim 10, the equivalent electrode layers of the various disks can be connected in a simple manner by axial electrical conductors, which are soldered onto the outside of the disk stack.
With the development of the invention according to claim 11, a voltage arc-over is reliably prevented between the electrode layers lying at different potential and connecting conductors, which are connected to the connecting sections.